Otto cycle is a thermodynamic cycle upon which a spark ignition engine works. Spark Ignition (or SI) engine uses petrol (or Gasoline) as fuel. Otto cycle was invented by Nicolas Otto in 1876.
Spark ignition engine is a type of internal combustion engines.
Below are P-V and T-S Diagrams of the Otto Cycle.
Otto Cycle is comprised of four processes (apart from the intake and exhaust process)
Process 1-2
This process comes after intake process. In case of SI engines, intake is a mixture of gasoline and air, also known as charge. In this process charge is compressed in isentropic manner.
Process 2-3
In this process the fuel gets blasted with the help of external spark (that is why the engine is known as spark ignition engine). This process is a constant volume heat addition process.
Process 3-4
After the combustion process, the exhaust gases expend. This process is an isentropic expansion process.
Process 4-1
After the expansion process, exhaust valve opens. This results into sudden drop of pressure inside engine cylinder. This process is a constant volume heat rejection process.
Note: Process 0-1 is the intake process and process 1-0 is the exhaust process.
Below is the table which shows heat and work interactions of the engine, along with the change in the internal energy.
| Process | Change in Internal Energy | Work Interaction | Heat Interaction |
| Process 1-2 | CV(T2-T1) | CV(T1-T2) | 0 |
| Process 2-3 | CV(T3-T2) | 0 | CV(T3-T2) |
| Process 3-4 | CV(T4-T3) | CV(T3-T4) | 0 |
| Process 4-1 | CV(T1-T4) | 0 | CV(T1-T4) |
Note: Negative value of heat interaction indicates heat rejected by the system and positive value of heat interaction indicates heat added to the system. Positive value of work interaction indicates work done by the system and negative value of work interaction indicates work done on the system.
Efficiency of Otto Cycle
Efficiency of the Otto Cycle is the ratio of work output to the heat input.
Work output = [CV(T3-T2)] – [CV(T4-T1)]
Heat Input = CV(T3-T2)
Efficiency = Work Output/Heat Input
After putting values of heat input and work output in the above formula, we get
η = 1 – [(T4-T1)/(T3-T2)]